In the related art, there is known a vehicle seat slide device which is disclosed in JP 2015-107792A (Reference 1), for example. The vehicle seat slide device is configured to include a lower rail, an upper rail, and a locking member. The lower rail is a first rail which extends in a front-rear direction of a vehicle, the upper rail is a second rail which is joined to the lower rail to be capable of moving, and the locking member is capable of restricting the movement of the upper rail.
In other words, as illustrated in FIG. 7, a lower rail 110 includes a pair of first side wall portions 111 which are lined up in a width direction, and a first connecting wall portion 112 which is separated from a upper rail 120 and connects base ends (bottom ends) of the first side wall portions 111 to each other. The lower rail 110 includes a pair of first flanges 113 which overhang to the inside in the width direction in which the first flanges 113 approach each other from the tips (the top ends) of the first side wall portions 111, and are further folded to the base end side of the first side wall portions 111.
The upper rail 120 includes a pair of second side wall portions 121 and a second connecting wall portion 122. The second side wall portions 121 are provided to line up in the width direction between the first flanges 113, and the second connecting wall portion 122 connects, in the width direction, the base ends (the top ends) of the second side wall portions 121 which distance from the lower rail 110. The upper rail 120 includes a pair of second flanges 123 which overhang to the outside in the width direction in which the second flanges 123 distance from each other from the tips (the bottom ends) of the second side wall portions 121, and are further folded so as to be surrounded by the first side wall portions 111 and the first flanges 113.
Both the lower rail 110 and the upper rail 120 have symmetrical (laterally symmetrical) rail cross sections in the width direction thereof, and are mainly prevented from dislocating in an up-down direction by the engagement of the first flanges 113 and the second flanges 123.
A locking member 130 is generally disposed inside the upper rail 120, and is joined to the upper rail 120 to rotate freely around an axial line extending in the width direction. Due to the lowering of a rear end portion of the locking member 130 together with rotation, the locking member 130 restricts the movement, and due to the rear end portion of the locking member 130 rising, the locking member 130 releases the restriction.
Incidentally, for example, when a significant load is input upward from the seat side (for example, a seatbelt) which is supported by the upper rail 120 due to a frontal collision of the vehicle or the like, as illustrated in FIGS. 8A and 8B, the opening end side of the upper rail 120 deforms to close to the inside in the width direction due to the upper rail 120 being pulled upward and meshing with the lower rail 110. At this time, when the locking member 130 which abuts (presses) against the second side wall portions 121 in accordance with the deformation is guided by the inclination of the second side wall portions 121 and has a tendency to move upward, or when the locking member 130 inclines such that a height difference arises in the width direction, it is possible that the original orientation may not be maintained.